The present invention pertains generally heaters used for the engine-independent heating of motor vehicles and for preheating engines and more particulary to a device for preheating fuel for an ultrasonic atomizer in a heater operated with liquid fuel, which includes a combustion chamber with a heat exchanger adjoining it and an ignition zone, into which the igniting device extends, as well as a burner head with means for supplying fuel and combustion air, wherein the ultrasonic atomizer has a central longitudinal bore for the fuel, which bore communicates with a radial bore for the fuel supply, and the ultrasonic atomizer is arranged within the burner housing in its longitudinal axis in an atomizer flange provided with an offset longitudinal through bore.
A device of this class is known from U.S. Pat. No. 4,732,322 (corresponding to West German Patent No. DE-PS 35,22,697). The device described in this patent is also used for the engine-independent heating of motor vehicles, construction equipment, and boats, as well as for preheating the engines of such units. Such vehicle heaters must be compact, because only very limited space is available for installation. Furthermore, they should have a low power consumption, because they are supplied with electricity from the vehicle battery, which has a particularly low power delivery capacity at very low temperatures, i.e., precisely when a particularly great need arises for the engine-independent heating of the vehicle's interior and, if desired, for preheating the engine.
Prior-art heaters with a heating capacity of circa 10 kW have a fuel throughput of circa 1.3 L diesel fuel per hour. In steady-state operation, i.e., under a hot running condition of the unit, 2 W electrical effective power is required for the ultrasonic atomization, including a safety reserve. This is generated by an ultrasound generator in an oscillator circuit and is fed to the ultrasonic atomizer. At cold start under extreme conditions, e.g., -40.degree. C., for which such heaters are designed in particular, the electrical power consumption increases 5-10-fold as a consequence of the greatly increasing viscosity of the fuel and the resulting increase in vapor deposition on the atomizer plate of the ultrasonic atomizer and the more difficult separation of the fuel mist drops from the fuel film, so that the ultrasound generator must deliver 10-15 W effective power for this state of operation.
For these reasons, the ultrasound generator in the prior-art devices is designed for two power levels, namely, for the cold start phase, e.g., for a power output of 12 W, and for normal operation with a power output of 2 W, level 1 being used only for the start-up range lasting a few minutes at an ambient temperature substantially below 0.degree. C. Once the operating temperature has been reached, so much heat is released by the combustion chamber of the heater by convection and radiation that it is possible to switch over to level 2 with its lower power output.
However, this prior-art device has the disadvantage that the ultrasound generator with its components, the output transmitter and the end-stage transistors, must be designed for the high output of circa 15 W, which leads to a reduced efficiency for the great majority of the operating time and is also expensive. Another disadvantage of this device is the fact that the high electrical effective power causes a great thermal and mechanical load for the ultrasonic atomizer during the cold start phase, which has unfavorable effects on service life and reliability.